Conventionally, ball clutches and roller clutches have been provided as a safety device between a driving part and a driven part of a torque transmission mechanism so that the driving part may rotate idly when an overload is encountered in the driven part. Thus, possible damage to the transmission may be avoided. The value of the transmitted torque when the torque shut-off takes place is generally called "tripping torque".
Typically in such an overload clutch, as shown in FIG. 8, torque transmitting elements 212 are held within hollow portions 224 formed in the driven plate 230 and are urged by means of the pressure plate 240 toward the recesses 232 formed in the hub 220 so as to effect torque transmission. The urging force is imparted by a spring 215. When an overload is encountered, the torque transmissing elements 212 escape from the recesses 232 in opposition to the urging force so as to shut off torque transmission.
The device shown in FIG. 8 is a so-called "automatic return type" in which the torque transmitting elements 212 which have escapted from the recesses 232 due to the overload condition, are adapted to automatically re-engage with the recesses 232 so as to resume the torque transmission once the causes of the overload are removed, since the torque transmitting elements are urged by the pressure plate 240 at all times.
However, in cases where the causes for an overload continue to exist relatively long, or limited rotations are unavoidable due to inertia of the driving or driven parts, the torque transmitting elements 212 must collide with the edges of the recessess 232, thereby causing shock or heat generation to the driving or driven parts. Also, due to such collisions, the edges of the recesses 232 are likely to be damaged, thereby causing fluctuations in the value of the tripping torque.
That is the reason why there is a need for a manually restorable overload clutch. The "manually restorable overload clutch" refers to an overload clutch in which the condition of torque transmission shut-off may be retained unless a manual and deliberate restoring operation is effected, once transmission of torque is shut off due to an overload.
Japanese Utility Medel Laid-open Application No. 132918/1984 discloses an example of such a manually restorable overload clutch, which is constructed as shown in FIG. 9 and has the following drawbacks:
1. Since the force urging the torque transmitting elements 312 toward the recesses 332 by means of the spring 315 is given by way of the tapered surfaces 301, 302 and 303, control of the tripping torque is rather difficult requiring accurate machining of those surfaces including their inclinations.
2. When the torque transmitting elements 312 escape from the recesses 332 and the ball 317 separates from the pressure plate 340, the ball 317 moves inwardly rolling on the tapered surface 303. When the ball 317 reaches the edge of the tapered surface 303, a force along the normal thereof acts on the ball 317. This force tends to force the ball 317 out of the moving path of the pressure plate 340 and increases by degrees. Thus, the torque shut-off takes place just when the ball 317 reaches the edge. Namely, not only the angles of the tapered surfaces, but the length of the slope is also important for determining the conditions of torque shut-off. By nearing the inclination of the tapered surface 303 to 90 degrees relative to the shaft and by nearing the inclination of the tapered surface 302 of the inner ring 311 to zero, it will be possible to decrease the amount of rolling travel of the ball 317 relative to the amount of movement of the torque transmitting elemens 312. However, in this case, a relatively large amount of travel of the pressure plate 340 will be required from the time when the ball 317 arrives at the edge until the time when it is forced out of the path of the pressure plate 340. Therefore, it will be practically difficult to realize a manually restorable overload clutch on the basis of the principle of the above prior art.
3. Since the urging force of the spring 315 also acts on the tapered surface 302 of the inner ring 311, a larger sized spring will be required as the inclination of the tapered surface 302 nears 90 degrees. As illustrated in FIG. 10a, the forces acting on the ball 317 comprises the spring force P11, the reaction force P21 of the pressure plate 340 and the reaction force P32 of the inner ring 311. The relationship between the axial and radial components of those forces may be represented as follows: EQU Axial components: P12=P22+P32 EQU Radial components: P23=P13+P33
As regards the axial components, the force by which the pressure plate 340 presses the ball 317 is smaller than the axial component force P12 of the spring 315. Consequently, it is unavoidable that a relatively large-sized spring is needed in case of the overload clutch illustrated.
In order to cope with the problem mentioned in the above point 2, the inclination of the tapered surface 302 of the inner ring 311 must be increased; however, the problem in the above point 3 calls it to be small, which are contradictory with each other.
4. FIG. 10b shows the various forces acting on the ball when the torque transmission has been shut off. In this case, there are the spring force P14, the reaction force P24 of the pressure plate 340 and the reaction force P34 of the inner ring 311. The relationship between the axial and radial components of those forces may be represented as follows: EQU Axial components: P15=P35 EQU Radial components: P24=P16+p36
Given .mu. as the coefficient of friction regarding the friction of the ball 317 on the surface of the pressure plate 340, the force F required to restore the pressure 340 so as to transmit torque will be represented as follows: EQU F=.mu. P24 EQU F=.mu. (P16+P36)
The force required for restoration is generated by the two tapered surfaces 301, 302 and as mentioned in the point 3 above, the spring must unavoidably be large, and consequently, P24 must also be large. As a result, the value of the force F becomes large rendering the restoration rather difficult.
5. Such an overload clutch 300 requires a large number of parts; and machining of the tapered surfaces 301, 302 and 303 is difficult.
6. In case of maintenance, the balls 317 fall off when the device is disassembled. Assemblage incorporating the balls 317 is also troublesome.
The object of the present invention therefore is to provide an overload clutch which is capable of solving the above-mentioned problems enabling accurate setting of tripping torque with a simple structure.